The Neem Tree

The neem tree (Azadirachta indica) has been valued by South Asians for thousands of years for, among other things, its ability to control insect pests. In the 1950s, when a visiting European entomologist noted neem trees were not fazed by a locust invasion, Westerners raised their eyebrows and began to take note.

Modern technology has helped us to understand this tree at the molecular level. All of the tree’s components—bark, leaves, wood and seeds—harbor biopesticides (naturally occurring substances that control pests) at some level. However, it’s the seeds—the small but powerfully concentrated packets of more than 70 potentially beneficial compounds—that have garnered the interest of researchers. South Asian people knew this and traditionally smashed the seeds, soaked them and sprinkled the resulting emulsion over their crops. This continues today, but over the last 40 years, a number of alternative extraction and separation techniques have been developed and patented in an effort to improve production capacity.

So far, azadirachtin appears to be the most abundant and, perhaps not coincidently, important neem compound when it comes to insect control. Its most notable trait is the ability to inhibit the synthesis and release of ecdysone, a steroidal prohormone that triggers the molting process in arthropods. In essence, it prevents insect larvae from pupating and ultimately causes their death. How effective azadirachtin is in blocking larval development depends on the concentration applied and how young the larvae are at the time of application. The younger they are, the more effective it is. Response varies among pest species as well. In addition to being a growth regulator, azadirachtin is an antifeedant. Munching pests will stop feeding on treated plants and become dormant. The compound can reduce pest fecundity as well, by interrupting communication between males and females, sterilizing them or simply deterring them from laying eggs on treated plants. For the most bang for your buck, though, your pest’s larval stage is the stage you need to monitor and target if you plan on using an azadirachtin-based product for control. Sprayed during the adult stage, it may reduce feeding and reproduction. Sprayed during the larval stage, it will kill.

More than 200 crop pests spanning several biological orders have been documented as being negatively affected by azadirachtin, either through its ability to regulate their growth, feeding, fertility, or a combination of the three. That list includes aphids, beetles, caterpillars, fungi, lace bugs, leafhoppers, leaf miners, mealybugs, mites, nematodes, psyllids, scales, snails, thrips, true bugs, viruses and whiteflies. Effective concentrations can range from as low as .1 to 1,000 ppm. Application rates for regulating larval growth in the field can be quite low, while those needed to regulate feeding are higher, typically over 200 ppm.

Clarified hydrophobic extract of neem oil, or neem oil, is a byproduct of azadirachtin extraction that also has merits. When sprayed, the oil coats and suffocates insects. It also repels certain insects and mites. Because it is an oil, it forms a barrier and prevents fungal diseases (i.e., mildews and rusts) from establishing. Residual solid matter from the extraction process is commonly called neem cake and sold as a fertilizer or a soil amendment. It has a higher NPK analysis than livestock manure and can inhibit soilborne fungal pathogens and parasitic nematodes. It has also been shown that when a plant’s roots absorb neem compounds, it can have systemic effects that have more holding power than if they were to be sprayed on the plant. For example, it has been shown that lettuce aphids can be significantly reduced when a neem-based product is applied through irrigation. This effect varies among plant species and with the amount fed to the plant. To make the best use of this trait, apply to seedlings as you transplant in the field and weekly for the next few weeks. Phytotoxicity can be a concern, though, and should be monitored.

When label directions are followed, neem products are not considered harmful to non-target organisms that do not feed on plant tissue. Adult insect predators, including spiders, earwigs, lady beetles, parasitoid wasps and ants, do not seem to be negatively affected by them, although there is evidence their immature stages are. Neem products are also considered harmless to pollinators, such as adult butterflies and bees, but product labels recommend not spraying it when honeybees are foraging. Repeated, highly concentrated sprayings can result in field bees carrying neem compounds back to the hive where they are fed to the developing brood. In most soil and aquatic environments, resident microbes will quickly metabolize these products, leaving no room for concern that they will accumulate over time. German research has shown earthworm growth and reproduction can actually benefit from the presence of neem products in the soil. Some product labels tell the applicator to avoid polluting water—fish kills can occur if large concentrations enter waterways.

From the grower’s prospective, neem products do have a shortcoming: they do not persist. Ultraviolet light causes them to quickly degrade and lose effectiveness within four to eight days of application. Leaf pH, temperature and rainfall can further reduce the effective period. They must be applied frequently, and that can be costly if significant acreage is being treated. A benefit is they can be used up to the day of harvest.

Seed quality and separation and extraction techniques vary. As a result, neem tree products vary as far as concentrations and effectiveness. One Canadian assessment of Indian neem oil samples revealed azadirachtin concentrations ranged from 50 to 6,800 ppm. If you’re interested in incorporating a neem product into your pest control repertoire, do a little research. It will help point you in the right direction as far as what product(s) are appropriate for your operation. Also, if you are USDA certified organic, make sure the product you are about to purchase is certified. Don’t assume it is: some neem products are approved for use on certified organic operations, others are not.

The author, a monthly contributor to Growing, is a biologist who lives and farms in Vermont’s Northeast Kingdom.